Cycles OpenCL and Mesa driver support
TL,DR
OpenCL will no longer be supported going forward.
Full Explanation
OpenCL development is stagnant and the implementation insufficient for a properly working render engine.
There are critical bugs and limitations that wont allow for a proper implementation using the API. Developer support is also limited with no significant new work being developed for a long time.
No further developments are expected at this point, and support will be dropped going forward, as stated in the Blender Code Blog about Cycles X

Deprecation
As part of the new architecture, we are removing some functionality.
Most notably:

OpenCL rendering kernels. The combination of the limited Cycles split kernel implementation, driver bugs, and stalled OpenCL standard
has made maintenance too difficult. We can only make the kinds of
bigger changes we are working on now by starting from a clean slate.
We are working with AMD and Intel to get the new kernels working on their GPUs, possibly using different APIs (such as CYCL, HIP,
Metal, …). This will not necessarily be ready for the first release,
the implementation needs to reach a higher quality bar than what is
there now. Long term, supporting all major GPU hardware vendors
remains an important goal, and we think that with this new
architecture we’ll be able to better performance and something
stability. It is just a matter of time until more GPUs are supported
in Cycles X again.


OpenCL for Cycles will be deprecated for the time being. Further developments are expected possibly using different APIs, but no definitive plans are made at the time of writing